1. Field of the Invention
The present invention relates to an X-ray CT scanner having a correcting function.
2. Description of the Related Art
An X-ray CT scanner is an apparatus which generates tomogram data by reconstructing, by using a computer, projection data obtained by irradiating an object to be examined with X-rays from the circumference of the object. These X-ray CT scanners are classified into the following three types in accordance with differences between the forms of X-ray beams.
The first one is a “fan-beam X-ray CT scanner” which radiates a fan-shaped X-ray beam from an X-ray tube. This fan-beam X-ray CT scanner acquires projection data by using an X-ray detector obtained by arranging about, e.g., 1,000 channels of detecting elements in a line. Projection data acquiring operation is repeated about 1,000 times while the X-ray tube rotates around an object to be examined. This fan-beam X-ray CT scanner is also called a “single-slice CT scanner” because data concerning a single slice are acquired.
The second one is a so-called “multi-slice X-ray CT scanner” in which several X-ray detectors each obtained by arranging about 1,000 channels of detecting elements in a line are juxtaposed in a slice direction. A slightly thick X-ray beam is radiated in accordance with the width of these juxtaposed detectors. This multi-slice X-ray CT scanner is so called because data of several slices can be simultaneously acquired.
The third one is a so-called “cone-beam X-ray CT scanner” in which a plurality of detecting elements each composed of a combination of, e.g., a scintillator and a photodiode are two-dimensionally arrayed. A conical or pyramidal X-ray beam is radiated in accordance with the width of these detecting elements in a slice direction. This cone-beam X-ray CT scanner is also called a volume X-ray CT scanner because volume data can be acquired at once.
The research of a cone-beam X-ray CT scanner has been advanced primarily on a system using an image intensifier (I.I.) as an X-ray detector since late 1980s. For example, in “Volume CT of anthropomorphic using a radiation therapy simulator” (Michael D. Silver, Yasuo Saito et al.; SPIE 1651 197-211 (1992)), the results of scan of chest phantoms in an experimental system combining a turntable and an I.I. are discussed. Some cone-beam X-ray CT scanners are beginning to be put into practical use as apparatuses for obtaining the shapes of high-contrast objects such as bones and blood vessels in angiography.
As described above, a cone-beam X-ray CT scanner has a wider divergent angle of an X-ray beam in a slice direction than in the other two types. In other words, the X-ray beam is thick on the rotation central axis. Since this increases the number of paths through which scattered rays reach detecting elements, the scattered ray amount increases. Scattered rays cause abuses, e.g., deteriorate the image contrast. This scattered ray increasing mechanism means that the scattered ray amount varies in accordance with a change in the beam thickness.
An X-ray CT scanner usually performs sensitivity correction in order to equalize the sensitivities of detecting elements. For this purpose, calibration data files (calibration data) for sensitivity correction are acquired by using a phantom (pseudo model). Since scattered rays change in accordance with the beam thickness as described above, these calibration data files must also be selectively used in accordance with the beam thickness.
This paradoxically means that the degree of freedom of beam thickness adjustment is limited by the types of calibration data files that the apparatus has.
Assume, for example, that a calibration data file acquired by a beam thickness X1 and a calibration data file acquired by a beam thickness X2 (>X1) are prepared. In this case, no corresponding calibration data files are prepared for beam thicknesses other than X1 and X2. Therefore, no such beam thicknesses can be set except when the inclusion of a scattered ray error is permitted.